Maybe not my all-time favorite (there are too many to choose from), but I will never forget the first time I launched Rocket League during a Steam free weekend. Back then they had Breathing Underwater as the main theme and the way the logo appeared in sync with the music was such a beautiful sight.

I linked the high quality song above, but if you want to experience exactly what I did, here's a link to a video with the intro captured from the game. Still gives me goosebumps when I see it.

Needless to say, I bought the game.